Highly efficient removal of pharmaceuticals from water by well-defined carbide-derived carbons

Chemical Engineering Journal 347 (2018) 595–606

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Highly efficient removal of pharmaceuticals from water by well-defined carbide-derived carbons

T

⁎

Silvia Álvarez-Torrellasa,b, , Macarena Munoza, Jan Gläselc, Zahara M. de Pedroa, ⁎ Carmen M. Domínguezb, Juan Garcíab, Bastian J.M. Etzoldc, , Jose A. Casasa a

Chemical Engineering Section, Sciences Faculty, Autónoma University, Cantoblanco, 28049 Madrid, Spain Chemical Engineering Department, Chemistry Sciences Faculty, Complutense University, Avda. Complutense s/n, 28040 Madrid, Spain c Technische Universität Darmstadt, Ernst-Berl-Institut für Technische und Makromolekulare Chemie, Alarich-Weiss-Straße 8, 64287 Darmstadt, Germany b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

removal of pharmaceuticals from • The water by CDCs and conventional carbons was investigated.

The specific surface area and hydro• phobicity determine the sorption capacity of the adsorbents.

pore size of the carbon materials • The plays a key role on the kinetics of the process.

combine high surface area with • CDCs high hydrophobicity leading to outstanding adsorptive behaviour.

showed a high adsorption • CDC-1000 capacity (550 mg g ) and short −1

equilibrium times (10 min).

A R T I C LE I N FO

A B S T R A C T

Keywords: Adsorption Carbide-derived carbons Pharmaceuticals Water treatment

In this work, we evaluate the feasibility of carbide-derived carbons (CDCs) as reliable and highly efficient adsorbents for the removal of pharmaceuticals from water. CDCs were produced from titanium carbide at temperatures within the range of 800–1400 °C employing chlorine as extraction agent. The materials were physicochemically characterized and tested in the adsorption of two relevant pharmaceuticals (diclofenac (DCF) and metronidazole (MNZ)) in aqueous solutions. We found that the specific surface area in combination with a low functionalized surface plays a key role on the adsorption capacity of the material while the average pore size determines the kinetics of the process. Notably, the CDCs showed an outstanding adsorptive behaviour compared to different conventional carbon materials under the same operating conditions, which was attributed not only to their high surface areas but mainly to their strong hydrophobic properties. The optimum material (specific surface area of 1676 m2 g−1) showed extremely high adsorption capacity values (551 and 410 mg g−1 for DCF and MNZ, respectively). Remarkably, the process was very fast (10 min of equilibrium time) even using low doses of adsorbent (0.3 mg mL−1). The performance of the optimum CDC was also demonstrated in bi-component systems while its versatility was demonstrated in a real wastewater treatment plant (WWTP) effluent, obtaining a good performance in terms of adsorption capacity (up to ∼400 mgDCF g−1) and kinetics (equilibrium

⁎ Corresponding authors at: Chemical Engineering Department, Chemistry Sciences Faculty, Complutense University, Avda. Complutense s/n, 28040 Madrid, Spain (S. ÁlvarezTorrellas); Technische Universität Darmstadt, Ernst-Berl-Institut für Technische und Makromolekulare Chemie, Alarich-Weiss-Straße 8, 64287 Darmstadt, Germany (B.J.M. Etzold). E-mail addresses: [email protected] (S. Álvarez-Torrellas), [email protected] (B.J.M. Etzold).

https://doi.org/10.1016/j.cej.2018.04.127 Received 23 February 2018; Received in revised form 17 April 2018; Accepted 19 April 2018 Available online 22 April 2018 1385-8947/ © 2018 Published by Elsevier B.V.

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time of 10 min). These results demonstrate the promise of CDCs for reliable, effective and fast flow-through tertiary water treatment.

1. Introduction

application of these materials in water treatment since their performance can vary strongly, directly influencing the efficiency of the process with the associated consequences for the environment. Furthermore, the leaching of metal ashes could be a source of additional water pollution. On the other hand, conventional ACs usually lead to slow pollutant uptakes (of the order of hours) from water. This is most probably due to the presence of very small pores within the very broad pore size distribution of these materials, acting as molecular sieves [32–34]. The application of innovative carbon materials of very high quality with reproducible textural and chemical properties would warrant reliable water treatment systems. In this context, carbide-derived carbons (CDCs) appear as promising candidates. They are produced by reactive extraction of the metal or metalloid atoms, transforming the carbide structure into pure carbon while maintaining the original shape and volume of the precursor. Thus, CDC properties can be reproduced with high accuracy from batch to batch since the carbon structure depends on the synthesis method, applied temperature, pressure and choice of carbide precursor. Accordingly, the pore structure can be tuned with precision, ranging from very narrow and ultramicroporous to mesoporous with broader distribution as also from extremely disordered to crystalline structures [35–38]. The textural properties of the CDC can be then tuned to optimize the adsorption process taking into account both kinetics and adsorption capacity [39]. The outstanding properties of CDCs have promoted their application in different fields such as catalysis [40–43], electrochemical capacitors [44,45], gas storage [46] and coatings [47]. CDCs have been also recently investigated as adsorbents in gas-phase processes [48–50]. Nevertheless, so far these materials have been scarcely investigated for liquid-phase adsorption processes, mainly for the purification of biofluids [37,51] and, to the best of our knowledge, they have not been tested so far as potential adsorbents for the removal of pharmaceuticals from water. The aim of this work is to investigate the feasibility of CDCs for the removal of two relevant pharmaceuticals in aqueous solutions. CDCs were synthesized by chlorination of titanium carbide at different temperatures within the range of 800–1400 °C to vary their textural properties. The materials were fully characterized and their adsorption capacity was evaluated and compared to that of commercial powdered carbons (PCs). The treatment of the bi-component mixture of both drugs was also tested to study possible competitive effects. As a proof of concept, the optimum system was finally tested using a real WWTP effluent as matrix to further demonstrate the feasibility of CDCs as reliable and efficient adsorbents for drugs removal in tertiary water treatment.

In recent years, and especially after the advancement of sophisticated analytical techniques, many pharmaceuticals have been identified at trace levels (ng L−1–μg L−1) worldwide in the aquatic environment [1]. Municipal wastewater treatment plants (WWTPs) are considered the main sources of these pollutants as they are not generally prepared to deal with these complex substances and thus, they are usually ineffective in their complete removal [1–3]. Despite the low concentration of drugs contained in those effluents, their continuous input constitutes an important environmental threat given their persistence and hazardous nature [4–7]. So far, there are no legal requirements for discharge of these ubiquitous and biologically active substances, but this scenario is expected to change in the next few years. The European Union (EU) has recently approved a watch list of 17 substances, among them 7 pharmaceuticals, for their monitoring in the EU-water basins (Decision 2015/495/EC) [8]. Those substances showing a significant risk will be potentially listed as priority pollutants. In Switzerland, the removal of micropollutants is mandatory since the beginning of 2016. Advanced tertiary treatments, final polishing stages prior discharge to the receiving environment, have been accordingly implemented at WWTPs to provide the reduction of at least 80% of the micropollutants content. The widely administered anti-inflammatory diclofenac (DCF) and broad-spectrum antibiotic metronidazole (MNZ) have been selected as target pollutants in this work due to their high persistence and hazardous character. Their removal at WWTPs are within the range of 21–40% and 25–54%, respectively [9,10]. There is evidence that prolonged exposure to DCF leads to impairment of the general health of fish, inducing renal lesions and alteration of the gills [11]. Moreover, its toxic effect can be considerably increased in combination with other pharmaceuticals. That’s why this drug has been included in the aforementioned EU-Watch list and Swiss regulation. On the other hand, MNZ is a proven mutagen in bacterial systems, it is carcinogenic to animals and genotoxic to human cells [12]. It is clear that optimization of WWTPs by including efficient tertiary treatments to create an effective barrier to micropollutants emission is social responsibility and a task of high priority. Successful results have been reported at lab scale for the elimination of a wide range of pharmaceuticals by Fenton-like processes [13–15], ozonation [16,17] and photocatalysis [18,19]. Nevertheless, these technologies are somehow limited by the expensive energy/reagents requirements and their effectiveness at full scale must be still demonstrated. In this context, adsorption onto activated carbon (AC) is regarded as the most feasible alternative nowadays. In fact, in Switzerland, most of the WWTPs have been optimized by including powdered activated carbon (PAC) adsorption processes whereas advanced oxidation processes have been scarcely put into practice [20]. ACs play a prominent role in the field of organic pollutants adsorption from water [21–23]. These materials are usually produced from carbonaceous or lignocellulosic sources, such as wood, fruit shells or stones, waste material, lignine, coal and petroleum pitch [24–27]. Their large specific surface areas, ranging from 500 to 1500 m2 g−1 (or even higher), high pore volume values (> 0.5 cm3 g−1) and low-cost have promoted their broad application [28–30]. Nevertheless, these materials have several deficiencies. The wide variety of precursors used for their synthesis unavoidably leads to fluctuating textural and chemical properties of the resulting AC, even for constant synthesis parameters [31]. Moreover, it usually contains large ash contents and volatile impurities. These circumstances pose a significant risk for the

2. Materials and methods 2.1. Materials Titanium carbide (TiC) powder was purchased from Alfa Aesar GmbH & Co KG with a purity > 99.5% and a mean particle size of 2 µm. Different carbon materials were also tested to compare their performance with those of the CDCs. Commercial PACs were supplied by Merck (PAC-M, ref: 102514), Panreac (PAC-P, ref: 121237) and Norit (PAC-N, ref: SX-PLUS). A powdered carbon black was supplied by Chemviron (PC-C, ref: 2156090). An own-prepared PAC from peach stones by chemical activation with H3PO4 (PAC-PS) [52] was also tested. All carbon materials were sieved to yield a particle size below 50 μm. Acetonitrile (HPLC Plus gradient grade) and acetic acid (99.5%) 596

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out on a Setsys 1750CS from Setaram Instrumentation coupled to an online quadrupole mass spectrometer (UGA, Standford Research Systems) and TGA Q500 from TA Instruments, respectively. The samples were heated from 20 to 1000 °C, at a heating rate of 5 °C min−1 in helium and air atmosphere, respectively. The morphology of the samples was studied by scanning electronic microscopy (SEM) with a Hitachi S-3000N microscope.

were obtained from Carlo Erba and Panreac, respectively. DCF and MNZ, both with a purity > 98%, were supplied by Sigma-Aldrich and used as received. The physical and chemical properties of these pharmaceuticals are collected in Table 1. The experiments were carried out with deionized water supplied by a deionizer equipment (ELGA, Veolia Water). 2.2. Synthesis of the CDC materials

2.4. Adsorption tests

The procedure followed in the synthesis of the CDCs has been reported elsewhere [41,61]. Briefly, the reactive extraction of TiC was carried out in a tubular alumina reactor using Cl2 gas (1.5 mol m−3 in He) as extraction agent, at a superficial velocity of 0.03 m s−1 and a reaction time of 5 h. Afterwards, the reactor was purged with He for 30 min. The materials were then treated with H2 at the reaction temperature for 30 min in order to remove residual chlorine and volatile metal chlorides from the pore structure. The extraction temperatures tested were 800, 1000, 1200 and 1400 °C, being the resulting materials denoted as CDC-T (°C).

Kinetic and equilibrium adsorption experiments were carried out in order to determine the equilibrium time and adsorption isotherms of DCF, MNZ and DCF-MNZ onto the CDC materials. Both kinetic and equilibrium tests were performed in a thermostatic bath at constant temperature (30 °C) and stirring rate (200 rpm). In the former, the pharmaceutical solutions (50 mL, C0 = 100 mg L−1) were treated with a low dose of CDC (0.3 mg mL−1). The concentration of the drug was followed until the equilibrium time was reached. In the latter, the initial concentration of the drug was also fixed at 100 mg L−1 whereas the dose of CDC was varied from 2.5 to 75.0 mg using a volume of 50 mL. Finally, the versatility of the system was carrying out using a real WWTP effluent collected at the Community of Madrid (Spain) as aqueous matrix. In this case, the initial concentration of DCF was fixed at 10 mg L−1 and the amount of CDC was varied within the range of 0.002–0.15 mg mL−1. Each run was performed by triplicate and standard deviation less than 10% resulted in all cases. The equilibrium adsorption capacity (mg g−1) was calculated by the following Eq. (1):

2.3. Characterization of CDC materials The pore structure of the samples was characterized by means of high-resolution N2 (77.4 K) and CO2 (273.15 K) adsorption experiments using a Quadrasorb sorption instrument (Quantachrome Instruments). Before each sorption measurement the samples were outgassed at 523 K for 3 h under vacuum (10−5–10−6 Torr). The mean pore size (MPS) was determined assuming slit pores using the specific pore volume (VP) value and the specific surface area (SBET) calculated using the BET equation from N2 sorption: MPS = 2 VP/SBET. The pore volume was calculated considering only the adsorption data. Pore size distributions of the samples were obtained following Density Functional Theory (DFT) methodology. Powder X-ray diffraction (XRD) patterns were recorded in a Philips X’Pert Pro MPD diffractometer operated at 40 kV and 40 mA, using Cu Kα radiation in the 2θ range from 10 to 80°. The point of zero charge (PZC) of the materials was obtained according to a method described elsewhere [62]. Raman spectra were recorded in a Horiba Jobin Ybon HR 800 micro-Raman equipment, using a HeNe laser. The laser power and the wavelength at the sample surface were controlled at 20 mV and 633 nm, respectively. Water contact angle measurements were performed with a drop shape analysis system (Dataphysics OCA 15 system) in the sessile mode at room temperature. Temperature-programmed decomposition (TPD) and oxidation (TPO) analyses were also carried

qe =

(C0−Ce ) ·V W

(1)

−1

where qe (mg g ) is the equilibrium adsorption capacity of the system; C0 (mg L−1) is the initial concentration of the drug; Ce (mg L−1) is the equilibrium drug concentration; V (L) is the volume of the solution and W (g) is the weight of adsorbent. To get further insights on the adsorption performance, the selectivity index in multicomponent systems was calculated using the following Eq. (2):

S1/2 =

D1 D2

(2)

being D1 and D2 the distribution coefficients of the components considered (DCF, MNZ or TOC). Besides the distribution coefficients solid to liquid (D, L g−1) were determined as the relation between the equilibrium adsorption capacity, qe (mg g−1), and the equilibrium

Table 1 Physical and chemical properties of DCF and MNZ. DCF

MNZ

296.2 4.0–4.2 4.5–4.8 0.97 × 0.96 0.354

171.2 2.4–2.6 −0.1–0.2 0.64 × 0.63 0.215

Molecular structure

Molecular weight (g mol−1) pKaa log Kowa Size (nm)b Volume (nm3)c a b c

References: [53–59]. Size calculated using the software TURBOMOLE v7.02 (geometry optimization with standard method B88-P86 and TZVP basis) [60]. Volume calculated using the software COSMOtherm (version C30_1701). 597

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aqueous concentration, Ce (mg L−1), by the next Eq. (3):

D=

Table 2 Textural and surface properties of the CDC materials synthesized at different temperatures.

qe (3)

Ce

Liquid samples were taken from the reactors and immediately analyzed. The CDC was previously separated by filtration using a PTFE filter (0.45 µm). DCF and MNZ concentrations were followed by high performance liquid chromatography, HPLC-UV (Varian, ProStar), using an Eclipse Plus C18 column (150 mm × 4.6 mm) as stationary phase. The analysis were performed at 270 nm using 10/90% and 57/43% (v/ v) mixtures of acetonitrile and acetic acid aqueous solution (75 mM) as mobile phase (0.80 mL min−1) for MNZ and DCF, respectively.

CDC-800 CDC-1000 CDC-1200 CDC-1400

Kinetic and equilibrium models were fitted to the experimental data by non-linear methods using the Solver Microsoft Excel v. 2013 software. The models were evaluated using the residual standard deviation (SD), expressed by Eq. (4): n

⎛⎜ 1 ⎞⎟· ∑ (q −q )2 i,exp i,cal ⎝ n−p ⎠ i

(4)

where, qi, exp is the experimental adsorption capacity, qi, cal is the calculated adsorption capacity, n is the number of experimental data and p is the number of parameters of the fitted model. 3. Results and discussion 3.1. Characterization of the CDC materials The textural properties of the CDCs were studied through N2 adsorption-desorption analysis. The isotherms and pore size distribution of these materials are depicted in Fig. 1. The specific surface area (SBET), pore volume (VP) and mean pore size (MPS) are collected in Table 2. According to the N2 sorption isotherms (Fig. 1a), the materials can be classified in two groups. In first place, those carbons synthesized at extraction temperatures ranging from 800 to 1000 °C followed type Ia and Ib sorption isotherms, respectively. The change in the shape and position of the knee indicates a broadening of the size from narrow micropores (CDC-800) to a broader range including wider micropores and possibly narrow mesopores (CDC-1000) [63]. In second place, when the synthesis temperature was increased up to 1400 °C (CDC1200 and CDC-1400), a significant increase in the mesoporosity of the materials was observed (type IVa isotherms). In the case of CDC-1400, a pronounced type H2 hysteresis loop was observed, corresponding to

Adsorbed volume (cm3 g-1 STP)

MPS (nm)

FWHMD1 (cm−1)

PZC

1534 1676 1267 446

0.69 0.80 0.84 0.90

0.90 0.95 1.33 4.04

164 135 78 48

9.2 9.4 9.9 10.0

b)

a)

500

4000

400 2000 300 800 600 400 200 0

200 CDC-800 CDC-1000 CDC-1200 CDC-1400

100 0 0.0

0.2

0.4

0.6

0.8

1.0

1

Relative pressure (p/p0)

10

Differential pore size distribution (m2 nm-1 g-1)

600

VP (cm3 g−1)

network effects. This shape indicates an enlarged size distribution of the neck width compared to a H2 hysteresis (steep desorption branch) [37]. The observations are supported by the pore size distributions. CDC-800 and CDC-1000 materials exhibited profiles showing mainly micropores below 1.5 nm (Fig. 1b). A slightly broader pore size distribution was observed for CDC-1200, with sizes up 4 nm, whereas CDC-1400 showed clearly the presence of mesopores ranging from 4 to 30 nm. The SBET of the CDCs slightly increased with increasing the extraction temperature from 800 to 1000 °C while higher temperatures led to a marked surface decay, reaching a value of only 446 m2 g−1 for CDC1400 (Table 2). On the other hand, consistent with the abovementioned results, the VP and MPS of the CDCs increased with the extraction temperature. These changes in the porous structure of the materials can be associated to the increased crystallinity developed in the CDCs by increasing the extraction temperature, evolving from amorphous carbon to graphitic structures [35,37,38]. These results are consistent with those previously reported in the literature for CDCs synthesized from many precursors, including TiC [37], where the specific surface area has been shown a bell-shaped temperature dependence, reaching maximum values at intermediate temperatures of 800–1000 °C. In the same line, the correlation between the halogenation temperature and the resulting pore size with smaller pore sizes at lower temperature is in good agreement with previous works [37]. The morphology of the CDCs was investigated by scanning electron microscopy. A well-developed and homogeneous structure was observed for all the CDCs, which indicates that they preserve the size and morphology of the TiC precursor (see Fig. S1 of the Supplementary Material for SEM micrographs). The carbon structure of the materials was investigated by XRD and Raman spectroscopy analyses. As expected, the increase on the extraction temperature led to an increase in the structural order of the CDC [64] (Fig. 2a). The (002, 20–30° 2θ) plane was almost negligible for the CDC-800 and CDC-1000 materials, indicating a highly amorphous structure. In contrast, both (002) and (100, 42–46° 2θ) graphitic planes were well-defined for CDC-1200 and CDC-1400 materials. These findings are in accordance to the Raman analyses (Fig. 2b). All CDCs showed two shaped overlapping peaks in the first-order Raman

2.5. Statistical evaluation of the kinetic and isotherm parameters

SD =

SBET (m2 g−1)

Pore size (nm)

Fig. 1. N2 adsorption-desorption isotherms (a) and pore size distribution (b) of the CDC materials synthesized at different temperatures. 598

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S

b)

G1

(100)

Intensity (arbitrary units)

(002)

a)

D1 CDC-1400 CDC-1400 CDC-1200 CDC-1200 CDC-1000 CDC-800

10

20

30

40

50

60

70

CDC-1000 CDC-800

1000

2000

3000

Raman shift (cm -1)

Angle (°2 Theta)

Fig. 2. X-ray diffraction patterns (a) and Raman spectra (b) of the CDC materials synthesized at different temperatures.

CDCs cannot be only explained by the low functionalization of their surface but also by the microtexture [68]. These results indicate that the wetting of the CDC surface is unfavorable, and thus the fluid will minimize its contact with the surface. Therefore, water would not compete for the available surface sites for adsorption, and thus, the efficiency of organics removal would be increased. These strong hydrophobic properties together with the high specific surface areas of the CDCs make these materials promising candidates for adsorption of nonpolar compounds from water-phases.

spectrum (disorder-induced (D1) and graphite (G1) peaks, at 1360 and 1600 cm−1), characteristic of amorphous carbons, but only CDC-1200 and CDC-1400 exhibited the S band (2700 cm−1), related with the crystallographic ordering of the graphitic structure [65]. As can be seen in Table 2 (see Fig. S2 of the Supplementary Material for peak deconvolution plots), the full width at half maximum (FWHM) of the D1 value, which is sensitive for the degree of graphitization [66], was significantly decreased with the extraction temperature, demonstrating the increasing degree of structural order of the materials. Consistent with these results, TPO analyses confirmed the extremely high thermooxidative stability of CDCs, typical of crystalline structures (see Fig. S3 of the Supplementary Material for derivative thermogravimetric profiles). In order to elucidate the surface functionalities of the carbon materials, the PZC was determined and TPD measurements were also performed. The PZC values are listed in Table 2, clearly showing that CDC materials present a basic character, which is in good agreement with previous works [35]. These high PZC values also demonstrate that no residual Cl is present in the surface of the CDC materials after the hydrogenation treatment. In fact, TPD analyses (see Fig. S4 of the Supplementary Material for experimental data) showed negligible physi- or chemisorbed compounds, excepting the loss of some volatile adsorbed species, e.g. water, up to 200 °C, confirming the very low functionalization of the CDCs. The hydrophobicity of the materials was further demonstrated by water angle contact measurements (Fig. 3). Values within the range of 142 to 147°, much higher than 90°, typical of hydrophobic solids, and close to 150°, considered threshold value for superhydrophobic surfaces, were obtained [67]. A slight increase of the contact angle with increasing extraction temperature of the CDC is observed, thus with increasing graphitic character as also lower surface functionalization. Nevertheless, the extremely high hydrophobicity of

3.2. Adsorption capacity The adsorption isotherms of DCF and MNZ onto the CDC materials synthesized in this work are collected in Fig. 4. The initial concentration of the drug was fixed at 100 mg L−1 whereas the dose of adsorbent was varied within the range of 0.05–1.5 mg mL−1. It is clear that the textural properties of the materials played a key role on the adsorption performance. The equilibrium adsorption capacity increased with increasing the surface area of the material (Table 2). Accordingly, the adsorption capacity of the materials followed the order: CDC1000 > CDC-800 > CDC-1200 > CDC-1400. The best performance was offered by CDC-1000 material (SBET = 1676 m2 g−1), with outstanding maximum adsorption capacity values for both DCF and MNZ drugs (qeDCF = 551 mg g−1, qeMNZ = 410 mg g−1). Apart from the extremely high specific surface area of this material, its high pore volume together with a mean pore size that allowed the entrance of the target pollutants into the pore structure must be noted to explain its outstanding performance. In general, MNZ adsorption capacity onto CDC materials was higher than that observed for DCF. This could be explained by the chemical structure of MNZ molecule, involving an imidazolium ring, which could establish specific interactions (associated

Fig. 3. Static water contact angle measurements on the CDCs synthesized at different temperatures. 599

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CDC-800

CDC-1000

CDC-1200

CDC-1400

a)

b)

600

500

500

400

400

300

300

200

200

100

100

0

qe (mg g-1)

qe (mg g-1)

600

0 0

20

40

60

80

100

20

40

60

80

100

Ce (mg L-1) Fig. 4. Experimental (symbols) and fitted (solid lines) data for the equilibrium adsorption isotherms of DCF (a) and MNZ (b) in deionized water onto the CDC materials ([drug]0 = 100 mg L−1; [CDC] = 0.05–1.5 mg mL−1).

24 mg g−1. To further corroborate the outstanding adsorption capacity of the optimum adsorbent (CDC-1000), we compared its DCF uptake with those achieved by different carbon materials under the same operating conditions. Commercial PACs with different physical and chemical properties were investigated ((Merck (PAC-M), SBET = 1019 m2 g−1), (Panreac (PAC-P), SBET = 931 m2 g−1) and (Norit (PAC-N), SBET = 1115 m2 g−1)). A commercial powdered carbon black (Chemviron (PC-C), SBET = 75 m2 g−1) and an own-prepared PAC from peach stones (PAC-PS, SBET = 1521 m2 g−1) were also tested. Extended characterization data of the carbon materials can be found in Table S3 of the Supplementary Material. All these materials showed the presence of important amounts of oxygen groups, especially those carbons with high specific surface areas (above 1000 m2 g−1). As example, up to 633 μmolCO2 g−1 and 2982 μmolCO g−1 evolved during TPD of PAC-PS, the highest functionalized material. Previous works in the literature have demonstrated that there is a good correlation between the oxygen content of the material and the amount of water adsorbed [73,74], which allows to demonstrate that conventional carbons were significantly less hydrophobic than the CDCs. In contrast, as has been previously explained, CDC materials showed a clean and hydrophobic surface, with nearly non measurable content of oxygen groups, which is not a common feature of carbon materials of high surface area. Up to approx. 4000 µmolO g−1 the PZC is a good indicator for the amount of oxygen surface groups, being more acidic with higher oxygen content [39]. This trend can be also seen in this study and the PZC of these carbons and CDCs decreases and functionalization increases in the following order: CDCs < PC-C ∼ PAC-M ∼ PAC-P < PAC-N < PACPS. Fig. 5 shows the DCF maximum adsorption capacity achieved with the conventional carbon materials and the CDCs together with their specific surface area and PZC values (the adsorption isotherms obtained with the conventional carbon materials are collected in Fig. S5 of the Supplementary Material). A volcano plot like behaviour can be observed, confirming that both specific surface area and PZC present a strong effect on the adsorption capacity of the carbon materials. The activation methods applied to develop the pore structure of conventional carbon materials usually involves the generation of large amounts of oxygen groups at the surface, leading to acidic surfaces. In this sense, the conventional carbon materials with the highest specific surface areas (PAC-N, PAC-PS) showed also the lowest PZC values. The acidic groups present at their surfaces lead to competitive effects with water molecules, avoiding the efficient adsorption of DCF. In this sense, the excellent adsorptive behaviour of CDCs can be then attributed not only to their high surface areas but mainly to the highly strong hydrophobic properties of these materials, avoiding unfavorable

with its quadrupole moment) within the CDCs inner surface. Given the different adsorption behaviours exhibited by the CDCs, several empirical adsorption models, e.g., Langmuir, Freundlich, Sips and Guggenheim-Anderson de Boer (GAB) equations, were applied to fit the experimental adsorption data (see Supplementary Material for details). The fitted adsorption data of DCF and MNZ onto the CDC materials are depicted in Fig. 4 and the corresponding adsorption parameters are collected in Tables S1 and S2 of the Supplementary Material. It must be noted that all models allowed to describe succesfully the experimental data. The Langmuir equation was selected as representative example for the fitting of the DCF and MNZ isotherms with the CDC-800 material. It assumes the adsorption as the formation of monolayer adsorbate on energetically homogeneous surface of the adsorbent [69]. This is consistent with the low mean pore size of this CDC (0.90 nm), close to the molecular dimensions of the pollutants (Table 1), which avoids the adsorption of more adsorbate layers. The representative fitting example for the rest of the CDCs depicted in Fig. 4 was successfully achieved with the GAB equation. The GAB model is frequently used in the fitting of experimental multilayer isotherms [70], which is the case of the CDCs produced at temperatures at or above 1000 °C. This change on the adsorption behaviour can be explained by the increase in the pore size of the material (Table 2), which allows the formation of several layers of the adsorbate. This effect can be more clearly seen in the isotherms obtained with the CDC-1400 material, which showed a clear sigmoidal trend at aqueous concentrations of the drug above 80 mg L−1, consistent with its larger mean pore size (4.04 nm). The plateau observed up to that threshold value indicates the monolayer completion whereas the sigmoidal trend can be related to the occurrence of multilayer adsorption and a vertically packing of the adsorbate in the active centers. This behaviour can be typically seen in mesoporous materials [70]. The maximum adsorption capacity values obtained with the CDCs of high surface areas are significantly higher than those previously reported in the literature for the removal of DCF and MNZ using different carbon materials of similar SBET values. Ahmed et al. [32] reported maximum adsorption capacity values for MNZ removal of 181 mg g−1 with seed-based microporous AC (SBET = 1677 m2 g−1). More recently, Bhadra et al. [33] found DCF maximum adsorption capacity values of 83 and 487 mg g−1 using commercial AC (SBET = 1016 m2 g−1) and oxidized AC (SBET = 704 m2 g−1). In the same line, Rakik et al. [71] gave maximum adsorption capacity values of 148 mg g−1 in the removal of DCF by commercial AC of higher surface area (SBET = 1470 m2 g−1). Hu et al. [72] investigated the removal of DCF using multi-walled carbon nanotubes modified by nitric acid (SBET = 190 m2 g−1), obtaining a maximum adsorption capacity of 600

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600

CDCs

order to that found in the adsorption capacity study. In this sense, a very fast removal of DCF was achieved using the CDC-1400 material, reaching ∼90% of its equilibrium uptake in 1 min, whereas only 50% was achieved with the CDC-800. These results can be explained by the different pore sizes of the CDC materials, which increase with increasing the temperature of synthesis (Table 2). In this sense, MNZ, which is a relatively small molecule compared to the pore size exhibited by all the CDCs tested (Table 2), was removed very fast in all cases. In contrast, the adsorption of DCF, with a remarkable bigger size (Table 1), close to the pore diameter of the CDCs produced at lower temperatures, was faster with increasing the pore size of the adsorbent. It can be then confirmed that the specific surface area plays a key role on the adsorption capacity of the material but it’s the pore size that determines the kinetics of the system. The experimental data were successfully described by the pseudosecond order kinetic model (see Supplementary Material for details). The resulting rate constants as well as the calculated adsorption capacity values are collected in Table 3 (see Fig. S6 of the Supplementary Material for the fitting of the experimental data). The kinetic values obtained followed the trend previously described whereas the calculated adsorption capacity values are in good agreement with the experimental data obtained in the equilibrium adsorption experiments. While the rate constants were quite similar for MNZ removal regardless of the CDC material used, the kinetics of DCF uptake increased dramatically with increasing the synthesis temperature i.e. the pore size of the CDC. It must be highlighted that the obtained pseudo-second order rate constants are remarkably higher than those found in the literature [32,33]. For instance, Ahmed et al. [32] reported pseudo-second order rate constant values within the range of 1 × 10−3–3.9 × 10−3 g mg−1 min−1 for the adsorption of MNZ (20–100 mg L−1) onto seed-based microporous activated carbon. More recently, Bhadra et al. [33] obtained rate constant values around 0.2 × 10−3 g mg−1 min−1 in the removal of DCF (100 mg L−1) using oxidized activated carbon. Probably the absence of very small pore necks when comparing these CDCs with other ACs as well as the low surface functionalization avoiding adsorbed water hindering diffusion might be a reason for the very fast uptake.

1750

500

1500

400

1250 1000 750

200 500 100

250

S

4

2

0

C

D

C

-8 00 C D C -1 00 C 0 D C -1 20 C 0 D C -1 40 0

-N

-P

PA

C

C PA

-P

C PA

C PA

PC

-M

0

-C

0

6

PZC

300

8

SBET (m2 g-1)

qe (mg g-1)

10

Fig. 5. Maximum adsorption capacity of CDCs and conventional carbon materials together with their specific surface area and PZC values ([DCF]0 = 100 mg L−1; [CDC] = 0.05 mg mL−1).

interactions with water molecules (hydrogen-bonded clusters formation), which is crucial for micropollutants removal from aqueous solution [75]. 3.3. Adsorption kinetics As has been previously shown, the high surface area of the CDCs, overall of those synthesized at 800 and 1000 °C, allowed to achieve extremely high adsorption capacities for DCF and MNZ removal from water. Although this aspect is crucial for the potential application of these materials, a fast removal of the drugs should be also warranted. To evaluate the kinetics of the adsorption process, a low dose of CDC (0.3 mg mL−1) was used for the removal of both drugs in deionized water. To our knowledge, this dose is the lowest reported for the adsorption of pharmaceutical compounds onto different materials, such as activated carbons [76,77], mesoporous silicas [78,79], zeolites [80,81] and novel polymers [82]. The typical values found in literature are within the range of 1 to 20 mg mL−1. All the CDCs were tested to analyze the effect of the textural properties of the material on the kinetics of the process. Fig. 6 shows the rate of DCF and MNZ uptake onto the CDC materials. For comparison purposes, the removal efficiency is depicted considering the equilibrium uptake of each drug onto a given CDC. As can be seen, the nature of the drug showed an important influence on the adsorption kinetics. MNZ was removed very quickly, reaching ∼95% of its equilibrium uptake in less than 5 min regardless of the CDC tested. On the opposite, the rate of DCF removal was increased with increasing the synthesis temperature of the CDC, following an inverse

It is clear that pharmaceuticals do not appear isolated in real WWTP effluents and thus, analyzing the treatment of their mixtures is essential. Despite this fact, few reports on multi-component adsorption of pharmaceutical mixtures have been found in the literature [22,83,84]. It is a complex issue where the interaction of the compounds, their molecular structures and initial concentrations as well as the physical

CDC-1000

CDC-1200

CDC-1400

100

100

80

80

60

60

40

40

20

20 b)

a)

0 0

10 20 30 40 50 60 0

Removal efficiency (%)

Removal efficiency (%)

CDC-800

3.4. Adsorption of bi-component (DCF-MNZ) mixtures

0

10 20 30 40 50 60

time (min) Fig. 6. Rate of DCF (a) and MNZ (b) uptake onto the CDC materials in deionized water ([drug]0 = 100 mg L−1; [CDC] = 0.3 mg mL−1). 601

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Table 3 Parameters of adsorption kinetics of DCF and MNZ onto the CDCs synthesized at different temperatures. DCF

CDC-800 CDC-1000 CDC-1200 CDC-1400

MNZ 3

2

qe exp (mg g−1)

qe cal (mg g−1)

k × 10 (g mg−1 min−1)

R

338.1 350.5 260.7 254.2

336.7 350.3 260.6 254.1

0.5 3.0 6.8 7.7

0.96 0.99 0.95 0.99

DCF

-1

k × 103 (g mg−1 min−1)

R2

318.1 334.5 260.0 240.5

317.5 334.1 259.5 240.0

7.9 8.7 8.8 8.6

0.99 0.99 0.99 0.99

MNZ CDC-1000

CDC-800

600

500

500

400

400

300

300

200

200

100

100

600

600

-1

CDC-1400

CDC-1200

500

500

400

400

300

300

200

200

100

100

0

qe (mg g )

qe (mg g )

qe cal (mg g−1)

CDC material, obtaining values close to 1 (see Table S5 of the Supplementary Material). According to previous works, the polarity effect is more pronounced for more dissimilar species [86]. With regard to the adsorption capacities exhibited by the different CDCs, the same trend followed in the single-component adsorption was observed. In this sense, the highest adsorption capacity was achieved by the CDC-1000 material due to its higher surface area while the lowest was that of CDC-1400. The extended-Freundlich model (see the Supplementary Material for further details), one of the most widely used for describing the adsorption of multicomponent systems [87,88], was succesfully fitted to the experimental data (Fig. 7) (see Table S4 of the Supplementary Material for the parameters obtained). The fitting was exceptionally good for the adsorption onto carbon materials synthesized at lower temperatures (CDC-800 and CDC-1000). Nonetheless, due to the larger pore size of the materials synthesized at higher temperatures (CDC-1200 and CDC-1400), multilayer adsorption became more important, and the deviations between the experimental and theoretical data were more significant (Fig. 7).

and chemical nature of the adsorbent must be considered. The adsorption capacity of the CDCs for the treatment of the bicomponent mixture was evaluated using an initial concentration of each drug of 100 mg L−1 and a CDC dose varied within the range of 0.05–1.5 mg mL−1. The resulting isotherms are collected in Fig. 7. For comparison purposes, the isotherms obtained in the adsorption of the isolated drugs is also shown. It is evident that mutual competitive effect between DCF and MNZ occurred in the binary system for all the CDCs tested since a reduction of the overall uptake capacity of both pollutants was observed compared to the single-systems. The reduction of MNZ uptake was around 36%, 52%, 43% and 23% for CDC-800, CDC-1000, CDC-1200 and CDC-1400, respectively, while the decrease observed in the removal of DCF in the binary system were 40%, 55%, 26% and 24%. These values were calculated by comparing the maximum adsorption capacity obtained for each drug in the single and binary systems. These results demonstrate that, despite their different polarity (Table 1), the drugs are competing for the same adsorption sites on the adsorbents [85]. In fact, the selectivity index (SDCF/MNZ) was calculated for each

600

qe exp (mg g−1)

0 0

20

40

60

80

100

20

40

60

80

100

-1

Ce (mg L ) Fig. 7. Experimental (symbols) and fitted (solid lines) data for the equilibrium adsorption isotherms of DCF and MNZ isolated (solid symbols) and in mixture (open symbols) in deionized water onto the CDC materials ([drug]0 = 100 mg L−1; [CDC] = 0.05–1.5 mg mL−1). 602

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observed a strong decrease on the adsorption capacity of commercial wood-based activated carbon in natural water (from 6 mg g−1 to 1 mg g−1) by varying the initial concentration of the target compound (atrazine) from 50 μg L−1 to 10 μg L−1 using an adsorbent dose of 3 mg mL−1. The presence of co-existing substances in the real WWTP effluent adversely affected the removal yield of the drug (Fig. 9a). The experimental DCF maximum adsorption capacity in WWTP effluent was decreased to 375 mg g−1. This reduction in the adsorption performance could be explained by different factors, such as competition for the adsorption sites, pores blockage or the interaction between the drug and organic species present in the effluent, as it has been described by other authors in the treatment of real matrices [23,90]. It is clear that TOC content is important to evaluate the effect of the WWTP matrix to the adsorption performance but, according to a recent work from Mailler et al. [90], the reduction can be due to adsorption competition and/or to interactions of micropollutants with non-adsorbable dissolved organic matter, maintaining them in solution. In particular, protein-like molecules were identified as the most problematic for adsorption competition in wastewater. Nonetheless, this deeper analysis of the WWTP effluent was not carried out in this work considering the low TOC content of the real water and also taking into account that the adsorption capacity value achieved was reasonable and even higher than those achieved by common commercial carbon adsorbents in deionized water (Fig. 5). To get further insights, the selectivity index of DCF related to the TOC present in the WWTP effluent was calculated following the Eqs. (2) and (3) DCF and TOC at equilibrium conditions ((qDCF = 375 mg g−1, qTOC = 9 mg g−1) for the corresponding equilibrium aqueous concentrations (CDCF = 7.5 mg L−1, −1 CTOC = 1.9 mg L )). Remarkably, a high selectivity index for DCF removal (SDCF/TOC = 10.7) was achieved. In general, adsorption selectivity index values higher than one indicate a successful separation of the target compound from the other species present in the aqueous medium [91]. Therefore, it can be confirmed that in this case the adsorbent showed an extremely high selectivity towards the pharmaceutical in the WWTP effluent. With regard to the kinetics, the decrease of pharmaceutical concentration did not affect significantly the equilibration time (Fig. 9b), which was achieved in less than 5 min using a dose of adsorbent of 0.15 mg mL−1, as occurred working with a DCF concentration of 100 mg L−1 under the same operating conditions (data not shown). This

It is evident that the adsorption of both DCF and MNZ was decreased when they appeared in mixtures due to competitive effects. Nevertheless, this aspect was less pronounced with increasing the pore size of the CDC material. To better appreciate this behaviour, the experimental adsorption capacity values exhibited by the CDC materials in both isolated and mixture solutions are summarized in Fig. 8. The aforementioned competitive effect among both drugs is clear in those materials of low pore sizes (CDC-800 and CDC-1000), reaching almost the same or even slightly lower adsorption capacity values (in total weight of drugs removed per mass of CDC) when both compounds appeared in mixture. Monolayer adsorption behaviour was observed in these cases (Fig. 7). On the opposite, those CDCs with higher pore sizes led to multilayer isotherms (Fig. 7). This is quite evident with CDC1400, with a considerably larger pore size that allows the adsorption of more layers of DCF and MNZ. Therefore, the maximum adsorption capacity when both drugs are present was quite higher than those achieved for the single compounds (Fig. 8). Similar conclusions were reached by et Mirzaei et al. [85], who investigated the binary adsorption of methyl tert-butyl ether and tert-butyl alcohol onto nano-perfluorooctyl alumina materials. They also found that the presence of large hydrophobic pores in the adsorbent are the dominant factor to achieve high adsorption capacities in binary systems as they can adsorb more layers of the adsorbates. The rates of DCF and MNZ uptake onto the CDC materials in binary systems were quite similar to those observed in the single-component study (see Fig. S7 of the Supplementary Material for experimental and fitting data). In general, the removal of MNZ was faster than that of DCF regardless of the CDC tested. Rate constant values for MNZ removal within the range of 4.1 × 10−3–4.3 × 10−3 g mg min−1 were obtained. On the opposite, the adsorption of DCF was highly affected by the textural properties of the CDC, obtaining rate constant values of 0.3 × 10−3, 0.7 × 10−3, 1.0 × 10−3 and 12 × 10−3 g mg min−1 for CDC-800, CDC-1000, CDC-1200 and CDC-1400, respectively. Remarkably, the equilibrium time was not longer than that observed in the single-component experiments. In this sense, equilibration times around 10 min were required with MNZ while ranges from 5 to 60 min were necessary with DCF, depending on the CDC used. 3.5. Proof of concept CDC-1000 has proved to be an excellent adsorbent in terms of adsorption capacity and kinetics for both DCF and MNZ uptake, isolated and in mixture, from deionized water. To further demonstrate the potential application of this carbon material in practice, a real WWTP effluent was used as aqueous matrix spiked with a lower concentration of the drug (C0 = 10 mg L−1) with the aim of operating at more representative conditions. DCF was selected as target compound for this study as it is candidate to be priority pollutant given its hazardous nature and thus, its complete removal from WWTP discharges must be warranted in the near future. The impact of direct competition on the adsorption of DCF due to the presence of co-existing substances in the real WWTP effluent (462 μS cm−1, [TOC]0 = 2.6 mg L−1, [IC]0 = 19.7 mg L−1) was assessed by performing isotherm tests varying the amount of CDC within the range of to mg mL−1. Fig. 9a depicts the resulting isotherm in the WWTP effluent. For reference, the isotherm in deionized water under the same operating conditions is also presented. In first place, it should be noted that the use of 10-fold lower concentration of the drug did not affect to the adsorption capacity of the CDC, reaching a maximum experimental value in deionized water of 609 mg g−1, which demonstrates the high versatility of this material. This aspect is crucial for its potential application at the varying drugs concentrations of WWTP effluents. In fact, this is not a common behaviour of carbon adsorbents, whose adsorption capacity is usually a function of the initial concentration in water: the lower the initial concentration, the lower the observed adsorptive capacity [23,89]. For instance, Knappe et al. [89]

Single component MNZ DCF Binary system DCF + MNZ

600

qe exp (mg g-1)

500 400 300 200 100

-1 C

D

C

-1 C

D

C

-1 C D C

40

0 20

0 00

00 -8 C D C

0

0

Fig. 8. Experimental adsorption capacities of the CDC materials synthesized at different temperatures for the removal of isolated DCF and MNZ (bars) and their mixture (symbols) ([DCF]0 = 100 mg L−1; [CDC] = 0.05 mg mL−1). 603

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Deionized water

WWTP effluent

600

100

-1

qe (mg g )

80

400 60 300 40

200

20

100 b)

a)

0 0

2

4

6

8

10

-1

Ce (mg L )

5

10

15

20

25

Removal efficiency (%)

500

0

30

time (min)

Fig. 9. Adsorption isotherms (a) and rate (b) of DCF uptake onto CDC-1000 in deionized water and WWTP effluent ([DCF]0 = 10 mg L−1; a: [CDC] = 0.002 – 0.15 mg mL−1 and b: [CDC] = 0.15 mg mL−1).

favorable behaviour, which indicates that mass transfer resistance is not controlling the adsorption process, is not usually found in adsorbent materials [92]. The highly ordered- and opened-porous structure of the CDC materials seems to be the responsible for this effect [37]. The adsorbent showed a fast removal of DCF regardless of the composition of the water matrix. Although it is clear that the presence of co-existing substances slightly affected the kinetics due to competitive effects, almost 90% of the equilibrium uptake was reached in 5 min in the real WWTP effluent (Fig. 9b). This fast adsorption would warrant the complete removal of the drug in real WWTP facilities considering that 45 min is the representative contact time in current PAC tertiary processes [90].

Acknowledgments

4. Conclusions

References

CDCs have proved to be outstanding adsorbents for the removal of pharmaceuticals from water. Their application has allowed to evaluate the effect of the textural properties of the carbon materials on the adsorption process, a fact that so far remained unclear due to the scattering and low reproducible properties of conventional activated carbons. It has been shown that two factors play a key role on the adsorption capacity of the material while the pore size determines the kinetics of the process. First factor determining the uptake capacity is the BET surface area and second one a low oxygen surface functionalization leading to a hydrophobic character. The comparison of the performance of the CDCs with different carbon materials under the same operating conditions has allowed to demonstrate that CDCs can combine high specific surface area with high hydrophobicity leading to outstanding adsorptive behaviour. The optimum material (CDC-1000) has shown an extremely high adsorption capacity (up to ∼550 mgDCF g−1) within a wide range of initial drug concentrations (10–100 mg L−1). Moreover, even using low adsorbent doses (0.3 mg mL−1), short equilibrium times (10 min or even lower times for MNZ) have been found. Remarkably, the system has been highly efficient even in bi-component systems, allowing to remove rapidly and simultaneously the drugs present in mixture. Finally, the versatility of the optimum CDC has been confirmed in a real WWTP effluent both in terms of removal uptake and kinetics, with equilibration times around 10 min, which is considerably lower than the representative contact time in current powdered activated carbon tertiary processes (45 min). These results demonstrate the potential of CDCs as reliable adsorbents for the efficient and fast removal of pharmaceuticals in tertiary water treatment.

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This research has been supported by the Spanish MINECO through the project CTM2016-76454-R and by the CM through the project S2013/MAE-2716. S. Alvarez-Torellas and M. Munoz thank the Spanish MINECO for the Juan de la Cierva-Formación (FJCI-2014-19916) and Juan de la Cierva-Incorporación (IJCI-2014-19427) postdoctoral contracts, respectively. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2018.04.127.

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[42]

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[46]

[47]

[48] [49] [50] [51]

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